One of the many emissions from fossil fuel power generation is sulfur dioxide. The sulfur in the sulfur dioxide originates as an impurity in many fossil fuels, particularly coal. Sulfur dioxide is a combustion product formed by reacting sulfur in the fuel with oxygen. Current environmental regulations require industry to reduce emissions of sulfur dioxide because it is a source of acid rain. Accordingly, the power generation industry has adopted technologies to “scrub” the sulfur dioxide from effluent combustion gases. These technologies are referred to as flue gas desulfurization (FGD) technologies. In brief, FGD technology is used to at least partially remove sulfur dioxide from combustion gas in a coal burning power plant by injecting a slurry comprising water and a lime or limestone material into a combustion gas exhaust stream. The lime or limestone reacts with the sulfur dioxide to produce solids comprising compounds containing calcium and sulfur. Subsequent reaction of these compounds with oxygen produces calcium sulfate or gypsum and residual waste materials. Often gypsum is removed and sold while leaving a waste stream that is usually landfilled.
For example, in a wet scrubbing FGD process, the limestone is pulverized and is added to water to form a slurry. The slurry is injected into an absorber through which the combustion gas exhaust stream flows. The limestone in the slurry reacts with the sulfur dioxide in the combustion gas forming a compound containing calcium and sulfur. Once the slurry passes through the absorber, the slurry returns to a water treatment process.
The water treatment process has many purposes. One purpose is to oxidize any calcium sulfite to calcium sulfate. Compressed air and sulfuric acid are added to the slurry at temperatures of between about 130° F. and about 150° F. to oxidize any calcium sulfite present. The calcium sulfate is removed from the water by filtering, such as with hydrocyclones and dewatering belts. The water is further treated, as described below, for reuse. The calcium sulfate, in a hydrated form known as gypsum, may be sold to drywall manufacturers.
Besides the gypsum, the FGD process produces a waste stream comprising fine, unsettled particles. Additional treatment of the water following removal of gypsum is required to remove the fine, unsettled particles. A secondary water treatment process includes addition of various polymers, which facilitate flocculation and settling of the particles. The fine, unsettled particles and polymers are then pumped into a settling tank or clarifier where the particles flocculate and then settle due to gravity. Other additives may include biocides to control the bacteria content. Once the flocculated particles settle, they are pumped through an additional dewatering step. After dewatering, the settled particles appear as an orange, paste-like sludge. Gypsum, by contrast, is a pure white material. The sludge is often referred to as OFS (orange fluffy stuff). The OFS is a waste product, or by-product, of the FGD process following removal of commercially pure gypsum. Typically, the OFS is landfilled.
The demand for electrical power is increasing. As the consumption of coal increases, the volume of the FGD waste stream (i.e. OFS) also increases. What are needed are methods and products for turning OFS into viable products.